Conventionally, a single laser beam has been used for the laser cutting of semiconductor wafers comprising a matrix of semiconductor chips. A laser singulation machine for the semiconductor industry typically uses one high power laser. Usually, q-switched solid-state lasers with infrared, green or ultraviolet emissions are used. The laser beam is focused onto the semiconductor wafer via mirrors and lenses to melt its material and separate its semiconductor chips. The laser power is adjusted by an external attenuator.
If multiple laser beams are to be used simultaneously for cutting a semiconductor wafer, the laser beam may be split into multiple beams from a single laser power source by means of a diffraction optical element (“DOE”) system. Beam splitters and other free-space optics are generally incorporated in such a DOE system to obtain a desired laser-splitting outcome.
Examples of existing singulation processes include grooving, dicing and stealth dicing. During grooving, a groove is formed on the semiconductor wafer to remove only a low-k top layer of the wafer, and the wafer is only separated in a subsequent step. In a dicing process, the laser beam ablates sufficient semiconductor wafer material to cut fully through a thickness of the wafer. During stealth dicing, the laser beam is focused in-between opposing surfaces of the semiconductor wafer to melt the wafer while avoiding surface damage to the wafer. For laser cutting using any of the above singulation processes, multiple passes of a laser beam for cutting may be required due to different wavelengths, pulse energies, repetition frequencies or pulse lengths, or different polarizations per beam. However, reliance on a single power source restricts the cutting efficiency of a laser singulation apparatus.
In relation to grooving, it has been recognized that single-pass grooving is not possible with a single high power laser. The grooving process requires different laser frequencies and pulse energies which must be applied during different passes. More specifically, a “trenching” pass typically requires high repetition frequency and low pulse energy but good spatial overlap between consecutive pulses, whereas the “grooving” pass requires the removal of a lot of material, such that high pulse energy and low repetition rate but larger distance between consecutive pulses is required. This two-pass process significantly slows down the singulation throughput of the machine
Moreover, DOE systems are generally expensive and require regular preventive maintenance. Diffractive beam splitters that are used in current DOE systems need sophisticated beam delivery optics with many lenses and a spatial filter. With the use of only a single high-power laser, the lifetime of the optical elements is limited.
Since the number of laser beams and the properties of each laser beam are essentially fixed by the optical path design of the DOE system, the laser beams cannot easily be controlled independently of each other because they are all derived from the same laser source. It would thus be beneficial to be able to independently control each laser beam so as to avoid the aforesaid shortcomings of the prior art when performing laser singulation.